Cemented carbide tools are made by consolidating extremely hard and extremely fine metal carbide particles together with a suitable binder or cement. Typical of such tools are those made of tungsten carbide cemented with cobalt. Although the tungsten carbide tools are the most widely used, hard carbides of other metals such as titanium, vanadium, chromium or molybdenum may also be used. All of the metals that produce hard carbides are relatively expensive metals.
Cobalt is the most widely used cementing material although other cementing materials may be employed, such as iron, nickel, chromium or molybdenum. Although the cementing materials are all metals, they will be referred to herein as coments; and their oxides will be referred to herein as cement oxides in order to distinguish them from the carbide-forming metals and metal oxides which will be referred to herein as metals and metal oxides.
When tools become worn or when tools are prepared with flaws that make them inadequate, it is desirable to reclaim the expensive metal carbide. Known methods for reclaiming metal carbides are either so expensive or so difficult to effect that they are not presently used; or, if used, the economy of such reclaiming processes is marginal. Among the known methods for reclaiming metal carbides are purely mechanical methods that involve extensive pulverization of the cemented carbides to reduce them to small enough particles so that they are separated from their cement and are capable of being reconsolidated. To reduce cemented carbides to such small sizes requires first milling them to a course powder and then injecting that course powder into a high velocity gas stream which directs the particles against a hard target, such as a metal carbide, where they shatter on impact. This process suffers from incomplete separation of metal carbides from the binding material; it uses expensive equipment; and it requires classification of the product according to size and recycling in order to recover significant amounts of the metal carbide.
Another technique employs liquid-phase zinc to dissolve the cement, thereby freeing metal carbide particles. This process must use large amounts of zinc and must circulate molten metal under a protective atmosphere to avoid oxidation. The process also requires treating the zinc-cement solution to recover the cement from the zinc, to recover the cement value, and to restore the zinc for reuse in the process. This process requires a great deal of expensive equipment and involves the difficulty of handling molten metal.
Another technique for reclaiming metal carbides is to dissolve the cement in an aggressive chemical medium such as boiling ferric chloride-hydrogen chloride solutions. Long time periods are required for chemical attack on the dense cemented carbide material to be effective. In addition, a great deal of special equipment must be employed to handle the aggressive solutions and to recover dissolved cement from them.
Another method for reclaiming cemented carbide materials is to treat them with a vapor phase mixture of chlorine gas, carbon monoxide and sulfur. This process is a slow process to effect, and it employs extremely dangerous gases which must be used with special caution and with special equipment.